The use of ultrasonically vibrated slender hollow metal tubes for cutting and removing biological tissue is well known. U.S. Pat. Nos. 3,589,363, 4,063,557, 4,223,676, 4,425,115, 4,493,694, 4,516,398, 4,750,902, and 4,750,902 disclose devices which utilize such tubes for the removal of such tissue.
In the prior art, the end of the slender hollow metal tube is vibrated axially by means of an ultrasonic transducer. When the tip is brought into contact with tissue, cavitation or mechanical shearing separates a core-like section of tissue which is then aspirated into the bore of the hollow tip by means of a vacuum provided therein. A separate suction pump which is connected to the tip provides the required vacuum. The tissue requiring excision such as, for example, brain neoplasms or prostatic tissue is usually soft and compliant. Tissue cores comprised of soft matter such as this flows easily through the tip and the tubing to which it is usually connected. Because of the compliance of the tissue, bends intentionally present in parts of the tip and tubing do not impeded the flow of such cores to the collection vessel. Generally, the diameter of the tip and associated tubing is of a constant diameter or cross section.
It is also known by U.S. Pat. No. 4,248,232 to utilize an ultrasonic tool for dissolving the bond between nested components such as a bone prosthesis and surrounding bone which are cemented together by a plastic layer. The vibrating tool, which in one embodiment may be a hollow probe that includes a thin tubule, is pressed into the bone cement. After the cavity of the tubule is filled, the probe is pulled out of the bone cement and the plastic is removed from the cavity. This patent also suggests that a window can be provided in the wall of the tubule so that bone cement traveling up the cavity can be continuously extruded and that a vacuum device can be connected to the window to continuously extract the plastic during use of the tool.
A new application of ultrasonic surgical cutting technology using an endoscopic ultrasonic aspirator has recently been developed by Wuchinich and is disclosed in U.S. patent application Ser. No. 07/529,029 filed May 25, 1990. This aspirator effectively removes thermoplastic cement adherent to bone surfaces during prosthesis revisional surgery. A particularly useful application for the technology is in the removal of methylmethacrylate from the femoral canal during hip revision surgery. A hollow metal tip is ultrasonically vibrated at a sufficient amplitude and frequency so as to melt thermoplastic surgical cement it contacts. Vacuum within the tip draws the melted cement into its bore as well as a large quantity of water utilized to cool the surgical site and protect against heat necrosis of adjacent cortical bone. Irrigation flow rates ranging from 100 to 500 cubic centimeters of water per minute are utilized for this protective purpose. As the melted cement is aspirated into the tip, it cools and recrystallizes so as to form a solid core. Irrigating saline enhances the rate of re-solidification within the bore of the tip.
Within a few millimeters of the surgical tip's cutting end, the cement has become a solid, rigid cylinder whose cross section duplicates that of the tip's bore. The length of these cylinders vary with the penetration depth of the tip into the cement, but can easily reach 10 to 20 times the diameter of the bore. Because the core, upon re-solidification, will substantially reproduce the configuration of the tip's bore, curves and irregularities, (such as recesses, burrs, protrusions and cavities), the internal surface of the tip may mold a curved or irregular core of plastic which may become an obstruction to the passage of the cores to the collection vessels. Once an initial core forms an obstruction, successive cores may quickly cause a complex blockage. Clearing such a blockage distracts the surgeon, prolongs the surgery and necessarily requires extension of anesthesia.
As a normal byproduct of ultrasonic transducer operation, a considerable amount of heat is generated. Removal of cement from the walls of a bone canal requires a prolonged operation of a transducer. The associated prolonged heat thereby generated may lead to cracks within, or a loosening of a structure housing the transducer.
The effectiveness of conventional ultrasonic devices for cutting and aspirating thermoplastic cement material can be further reduced or even neutralized by the adhesion of recrystallized cement to the exterior surface of the vibrating tip. Specifically, when the outer surface of the tip penetrates the cement to perform excavation, the cement may resolidify thereon even during brief periods of non-vibration. The resulting solid layer of cement material envelopes the tip and prevents reinitiation of tip vibration. If the static load imparted by the resolidified layer exceeds the power capacity of the electrically operated ultrasonic generator while the tip is embedded in the cement material, vibration will cease and the use of manual tools to dislodge the tip from the cement may be required.
Another undesirable load on the ultrasonic generator is that in addition to delivering the vibratory power needed to melt the cement material that is to be removed through the bore, it must also supply sufficient power to melt any cement material contacting the outer tip surface as it enters the cement. Any additional power thus consumed in melting the cement contacting the outer tip surface is converted directly into heat and results in an undesirable elevation of temperature at the excavation site. It has been found that undesirable power consumption is greatest for those tip surface configurations which utilize a uniform diameter or a diameter that progressively increases in the direction of the handpiece. Uniform or progressively tapered outer surface configurations also tend to impede the flow of cooling fluid to the excavation site because melted cement surrounding the outer tip surface restricts the flow to the end of the tip. Consequently, unwanted elevations in temperature are produced where the cement is being withdrawn, resolidification of the cement within the bore of the tip is retarded, and those portions of the tip most subject to the cyclic stress of vibration receive little or no cooling.
What is needed, therefore, is an ultrasonic surgical instrument for the removal of prosthetic cement which includes a hollow tip which is highly resistant to blockages caused by core jams, which minimizes the power required to penetrate the cement, which provides cooling fluid with access to the excavation site, and which reduces the likelihood of tip removal problems. What is also needed is an ultrasonic surgical instrument which includes an ultrasonic transducer which may provide prolonged operating time without suffering failure due to excessive heat production.